I understand the proceedure of putting a pad of clay on top of the piston, bolting down the heads, turning the engine over, and then measuring the thickness of the valve to piston clearance in the clay. ---- Can some one tell me what the minimum measurement you would want to have is?

(I'm not doing an engine right now. This just came up in a conversation I had yesterday. It was regarding a Ford engine.)

I understand the proceedure of putting a pad of clay on top of the piston, bolting down the heads, turning the engine over, and then measuring the thickness of the valve to piston clearance in the clay. ---- Can some one tell me what the minimum measurement you would want to have is?

(I'm not doing an engine right now. This just came up in a conversation I had yesterday. It was regarding a Ford engine.)

Thanks!

Not knowing what you've got in the engine at this moment, the proper procedure is to measure this with a solid lifter, to insure that no variance can occur as can happen with a hydraulic lifter as the valve spring is compressed.

Generally for engines with steel connecting rods .080 is considered sufficient for an intake while the hotter operating exhaust is usually set at .100 inch. Engines using aluminum rods should have .020 to .030 inch more clearance added to the above numbers.

More clearance than necessary is safer than less. But you also need to keep an eye on the thickness of the piston crown below the eyebrow clearance cuts to insure this doesn't get too thin as to result in structural failure of the piston crown. Remember that you can also adjust this clearance with the thickness of the head gasket if some compromise becomes necessary to preserve piston crown thickness. A cast piston needs more crown thickness than a forged version.

I understand the proceedure of putting a pad of clay on top of the piston, bolting down the heads, turning the engine over, and then measuring the thickness of the valve to piston clearance in the clay. ---- Can some one tell me what the minimum measurement you would want to have is?

(I'm not doing an engine right now. This just came up in a conversation I had yesterday. It was regarding a Ford engine.)

Thanks!

Seems confusion has developed about what you're actually measuring. Like all engine builder processes this one requires you put the engine together to some extent and take it apart again.

The process requires:

1) That the crank assembly be installed as well as the cam and cam drive such that the cam is properly timed to and rotates with the crank.

2) An example of the head gasket to be used is needed.

3) A pair solid lifters appropriate to the cam roller of flat. If hydraulic, this can be made by disassembling a hydraulic and filling its body with washers sufficient to allow installation of the cup with zero to .020 inch clearance between the top of the cup and the bottom of the retainer clip. This will simulate minimum operating clearance.

4) A cylinder head with at least one set of valves in it, they can use testing springs instead of the real thing. A full up check would be with a full set of lifters and both heads. For less than a full out race engine, an example cylinder should be sufficient.

5) Pick a cylinder to test #1 being the obvious choice. Apply a coating of light oil to the valves to prevent the Silly Putty from sticking to the valves but it will to the dry piston. Apply a bead of putty to the piston in the area below the valves The piston should be down the bore on the power stroke such that bolting the head on and connecting the valve train will not immediately crush the putty. The head is bolted in place and properly torqued.

6) A proper length pushrod connects the tappet to the rocker, if a solid lifter cam proper lash should be included. If a hydraulic lifter-ed cam adjust till there is just detectable drag on the push rod when you spin it with your finger tips. (This is a good opportunity to determine push rod length requirements if this hasn't been done yet, by using testing springs, you can use adjustable pushrods made for this purpose.

7) Rotate the engine in the clockwise direction when standing in front of it, two full revolutions plus enough to pass TDC the second time. This will cycle the cam fully thru its sequence relative to piston position, therefore, will imprint the putty with the absolute minimum clearance.

8) The head gets unbolted and removed.

9) The piston is brought back to TDC and the sample clay carefully removed and placed on a flat surface for dissection and measurement. Since the numbers you're looking at are fairly big when the sample is cut to expose the crushed area a machinists rule graduated in decmal inches is sufficient to measure with. Or if you have a caliper digital makes this simple read it off the backside from the slider. A micrometer can be used, but is overkill.

10) If everything checks out, take the modified lifter apart, dump the washers out and reassemble it before you forget.

You use a dial indicator on the retainer of a fully assembled, as run valve train ( no light weight checker springs) with the degree wheel installed you start 20 deg's BTDC and check in 2 degree increments until you find the closest point.

You need a tool that spans the rocker and has a handle that can pull the rocker open until the valve touches the top of the piston...

The safe numbers all over the internet are .080" intake and .100" exhaust.

Regarding the clay method I have a question. In order to do the clay method you have to install your head gaskets and then torque them to spec prior to rotating your motor. Correct?

If that is the case do you all buy two sets of head gaskets? If I'm not mistaken head gaskets have metal crush rings around the pistons and once they are crushed they are spent and if you remove the head again. (In order to remove the clay) Then the gaskets are junk. If I'm wrong please let me know!

As for the dial indicator. I don't understand how that will tell you your clearance... It can measure your lift but how will it measure the clearance between the valve & piston? The method I've seen used was to install two valves with a very light weight springs, just strong enough to keep the valve closed. You can then rotate the engine through two revolutions stopping every few degrees and you can then slide a feeler gauge between the rocker & valve stem. This way you can check the clearance anywhere in the cycle and you don't trash a set of head gaskets!

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